What width of tyre can I fit?

AllezAllezAllez

I've been looking at the Planet X XLS and it comes with "Mach 1 820 Rims On Deore M525 29er Wheelset / 29er / Black" and 35mm tyre's.

For road use what is the smallest width I will be able to run? WIll I be able to run 23c?

Monty Dog

Measure the external width of the rim - don't fit anything narrower. If you want a better ride and handling, fit 25 or even 28mm, if you want to go slower, have more punctures, a harsh ride and sketchy handling, stick with 23s.